In U.S. Pat. No. 3,181,098, a water-sensitive resistor for use in soil is disclosed. The resistor consists of a thin circular layer of suitable, granular, water-absorbant, fill material that is compressed by a sheet of metal into firm contact with a ceramic membrane through a screen electrode. The liquid in the fill is in molecular-diffusional contact with the soil solution through the membrane thus transmitting to the fill the matric suction of the liquid phase of the soil.
The electrolytic resistance of the above-described type of water-sensitive resistor has a stable responsive relation to the matric suction of the soil, at any given temperature and soil salinity condition. As the matric suction of the soil increases, the liquid content of the fill decreases and the electrolytic resistance of the fill between the electrodes increases. However, this resistance also increases if the temperature or salinity of the soil decreases. This negative response is a distinct disadvantage if an uncompensated water-sensitive resistor is used for control in irrigation, either automatically or manually, because plants have less need for water if the soil temperature or salinity decrease. In other words, decreases of soil temperature or salinity actually lessen the need for irrigation, but produce the effect in an electrolytic-resistance type sensor of an increase in matric suction, signalling an increased need for irrigation.
This aberration was substantially completely solved through the invention disclosed and claimed in U.S. Pat. No. 3,782,179. In that disclosure, it is taught that an apparatus for measuring the matric suction of soil could be fabricated which is free from the adverse effects due to changes in the salinity and temperature of the soil by employing two water-sensitive resistors one of which having a relatively coarse pore size distribution causing it to have a relatively large fractional change in resistance for an incremental change in matric suction while the other porous body having a relatively fine pore size distribution causing it to have a relatively small fractional change in resistance for an incremental change in matric suction.
Although the device described and claimed in U.S. Pat. No. 3,782,179 functions adequately for its intended purpose, certain difficulties have been observed particularly regarding the establishment and maintenance of electrical contacts across the water-sensitive resistors. As the reference patent discloses, electric contact across these resistors is most advantageously accomplished by providing grid or screen electrodes in a physically abutting relationship to parallel flat surfaces of the resistors. The screens or grids can be formed by a sputtered metal film which is, in turn, electrically connected to wires emanating from the device.
Unfortunately, unless gold or an equivalent noble metal is employed for fabricating electrodes from thin films, the electrolytic effect to which the electrodes are subjected causes a rapid and accelerated corrosive deterioration which greatly reduces the operating life of the device. In that gold and equivalent metals are prohibitively expensive, a serious drawback remained in practicing the invention as described and claimed in U.S. Pat. No. 3,782,179.
It is thus an object of the present invention to describe an apparatus for measuring the matric suction of soil which is free of the disadvantages outlined above inherent in the prior art.
It is yet another object of the present invention to describe an apparatus for measuring the matric suction of soil which possesses a usable functional life greatly extended as compared to similar apparatus taught by the prior art.
These and further objects of the present invention will be more readily appreciated when considering the following disclosure and the appended drawing which is a cross-sectional view of the apparatus of the present invention.